The differences between a bipolar arrangement and a monopolar arrangement can be shown most easily using the equivalent circuit diagrams that are provided in FIG. 1.
The bipolar arrangement corresponds to the upper part of the figure, which shows a series connection of individual voltage cells. The cells are placed together in such a way that a plus pole and minus pole are opposite each other in each case, which allows a simple switching of the voltage cells. This is the arrangement used, for example, to position the chambers of a lead accumulator, to insert the batteries into a torch or to position the fuel cell units in a bipolar stack.
The monopolar arrangement is that shown in the lower part of the figure. With this arrangement, the cells are placed together in such a way that two plus poles or two minus poles are opposite each other, in pairs, each time. In order to switch the individual voltage cells in series here, a comparatively complicated switching is necessary, and so such an arrangement of individual cells in most applications would not have any advantage over the bipolar arrangement. However, this does not apply for fuel cell stacks: advantages can also be achieved here with a monopolar arrangement, as will be explained below referring to FIG. 2.
Within a bipolar stack, every cathode has two neighbouring anodes and must be sealed against these to prevent fluid transfers. Within a cell, this is done in the interaction of electrolyte material (inside) and seals (outside). The separation between the neighbouring cells is made in each case through a separating plate, one side of which forms the cathode chamber and the other side of which forms the anode chamber. The fluid passages in the separating plates are formed in such a way that the cathode fluid from the one side (cathode side) does not come into contact with the anode fluid from the other side (anode side).
In a monopolar arrangement, on the other hand, anode and cathode pairs are formed. Indeed, the cathodes and anodes within such a pair must be electrically insulated from each other, but the fluid regions themselves do not need to be separated as long as the electrical conductivity of the cathode and anode fluids remains negligible (which is generally the case in spite of a certain conductivity, especially of the anode fluid). Two cathodes can therefore be grouped together in each case into one cathode chamber, and two anodes into one anode chamber.
FIG. 2 shows an exploded view of such an anode chamber. An arrangement of this type is described, for example, in DE 100 40 654 A1. An electrically insulating frame element 1 is sandwiched between two current collectors 2. The current collectors in turn border on electrolyte devices (MEA) (not shown in the figure). The current collectors 2 are used for current removal at the MEA-current collector interface, but at the same time should not noticeably reduce the contact surface of the anode fluid with the MEA: for this reason, the inner region of a current collector 2 is bridged with thin transverse webs which are sufficiently wide and numerous for current removal, but are so narrow that the active contact surface of the fluid with the MEA defined by the recesses 4 is not substantially reduced.
To guide the flow along the stack axis, four bore holes 5 are provided in the frame region (in the corners in this case) of the electrical conductors, whereby in each case two diametrically opposed bore holes serve to guide the anode fluid and the cathode fluid. Corresponding bore holes 6, 7 are also provided in the frame element 1. The bore holes 7 are connected via openings with the inner region 8 of the frame element 1. The supply of anode fluid into the inner region 8 is via one of the two bore holes 7, and the removal via the other, diametrically opposed bore hole 7. The inner region 8 represents the main volume of the anode chamber of the cell shown, since the thickness of the frame element 1 is far greater than that of the current collector 2.
The statements made here for the anode chamber also apply correspondingly for the cathode chamber. With a cathode chamber, the central frame element has recesses at the two other diametric fluid openings. For this, the sketched type of frame elements 1 only needs to be turned around, so that only one type of frame elements is necessary for the building of the stack.
In the arrangement shown, the flow through the corner regions, which do not exhibit any intake or outlet, is much less than, for example, that through the central region of the anode chamber. In order to achieve a more even flow distribution within the chambers, net-like insertions (not shown in the figure) can be provided in the anode and cathode chambers. But even with these, the fluid exchange in the stagnation regions of the anode and cathode chambers is lower than along the intake-outlet section, so that the active surface available cannot be put to the best use. Stagnation regions of this type form in particular if the stack is not favourably positioned in terms of space.